Fróðskaparrit - 01.01.2001, Page 132

Fróðskaparrit - 01.01.2001, Page 132
136 BREEDING DISTRIBUTION AND NUMBERS OF WREN (TROGLODYTES TROGLODYTES) IN THE FAROEISLANDS 1978/79, dropped by about 75% from a pre- viously rather stable level at about 50 males, but since 1981 (10 pairs) the number has been increasing (Dymond, 1991). For St. Kilda, where many census-takings have been attempted over the years, it has been claimed that the number of pairs in the ”Village area” remained stable over a long run of years and that this reflected the situ- ation in the rest of the population (Fisher, 1947; 1948). This was later refuted by Williamson (1958), who claimed a subse- quent, decreasing trend in the "Village area” and argued for fluctuations also in the population breeding in the cliffs (about 85% of total population). Also from Ice- land fluctuations have been reported (e.g. Bengtson, 1970). As to the Faroes, the an- nual fluctuations reported for the four study plots (Tab. 2) represent the only informa- tion available. The plots were selected for special studies of colour-banded individu- als, and thought to represent wren strong- holds. Therefore the plots should, accord- ing to the theory of ”ideal-free distribution” (Fretwell and Lucas, 1970; Fretwell, 1972) exhibit less variation than more marginal areas with lower densities. The variation (CV) for the four plots and the pooled num- bers all fall in the range 15-28% (Tab. 2), which is considerably less than the 63% variation recorded in Sweden (Nilsson, 1986), and lower than the about 45% calcu- lated from data from Iceland (Bengtson, 1970). It is not the purpose of this study to discuss the population dynainics of the wren in the Faroes but to note that in favourable wren-areas in the Faroes the magnitude of annual flucuations usually appears to be in the order of 15-30%. Whether this holds also for less favourable areas and for the wrens breeding on the sea cliffs is not known. During the course of present study, i.e. the past 20 yr, no single years stand out as being particularly ”good” or ”poor” with respect to the number of breeding wrens, nor is there any obvious trends in numbers. The previous total estimates at 250-500 and 500 pairs (Bloch and Sørensen, 1984; Bloch et ai, 1996) most likely underesti- mated the true numbers and should not be taken as evidence for a recent increase. A literature search for other evidence of fluc- tuations or long-term changes produced lit- tle of value. Most authors refer to the wren as "common” or ”abundant” without giving any numbers. Dare (1966), who made a fortnightly visited to several islands (v/z. Streymoy, Eysturoy, Vágar, and Nólsoy) in Maj-June 1965, found the wren to be un- common with < 10 records. Williamson (1953) visited Streymoy, Nólsoy, Skúvoy, and Stóra Dímun in late July and early Au- gust 1947 and found the wren rnore abun- dant than in any of the years 1942-1945, in spite of exceptionally harsh weather condi- tions in the early months of the year. On Mykines in 1900 the wren occurred ”in no small numbers” and in the following year it was found ”in rather few numbers; fewer it seems than in the previous year” (Ander- sen, 1901; 1902); not stating whether it refers to breeding birds or includes young (in the Faroes, recruitment, and thus au- tumn and early winter numbers, may vary considerably between years according to pers.obs.). Two published accounts, dating
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